The present invention relates to measuring noise power within a narrow frequency range. More particularly, the invention concerns measuring and compensating for inaccuracies in the measurement frequency used by a noise power meter.
Accurate calculations of a device under test's (DUT's) noise parameters require noise power measurements to be taken at the frequency at which its scattering parameters were measured. Conventionally, noise power meters measure "spot noise power," that is, noise power contained within a relatively narrow frequency range. The center of this frequency range is called the "measurement frequency." A noise power meter typically isolates signals within the narrow frequency range using a variable frequency conversion stage followed by a fixed narrow passband filter. The variable frequency conversion stage shifts the frequency of the measured signal by a selected amount so that it lies within the narrow passband of the bandpass filter. The noise power meter measures the power of the signal at the bandpass filter's output.
Any inaccuracy in the variable frequency conversion stage or in the bandpass filter results in a corresponding error in the measurement frequency. These errors can be eliminated if the noise power meter's variable frequency conversion stage uses an accurate frequency synthesizer as its local oscillator and the bandpass filter is perfectly centered on the desired measurement frequency. However, conventional noise power meters use imperfect bandpass filters and less expensive oscillators instead of frequency synthesizers, with corresponding frequency inaccuracies.
Inaccuracies in the measurement frequency can become significant when the DUT has a gain or noise power which changes dramatically with its connected source admittance. Since the source admittance presented to the DUT varies rapidly with the measurement frequency (see Larock and Meys, "Automatic Noise Temperature Measurement Through Frequency Variation," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-30, No. 8, August 1982.), an error in the measurement frequency changes the source admittance, which in turn changes the DUT's gain. These changes in the DUT's gain cause the measured noise power to be substantially different from the DUT's noise power at the desired measurement frequency.
What is needed, then, is a method of measuring and compensating for inaccuracies in the measurement frequency of noise power meters, allowing a DUT's noise power to be measured at the frequency at which its scattering parameters were measured.